1. Field of the Invention
The present invention relates to a fuel pump for drawing in a fuel such as gasoline etc., increasing the pressure thereof, and discharging the pressurized fuel.
2. Description of the Related Art
A known fuel pump generally comprises a substantially disc-shaped impeller and a casing. The impeller is rotatably disposed within the casing. A group of concavities is formed in an upper face of the impeller, and another group of concavities is formed in a lower face of the impeller. The concavities are repeated in a circumferential direction. Adjacent concavities are separated by partition walls that extend in a radial direction. A first groove is formed in a front surface of the casing in an area directly facing the group of concavities in the upper face of the impeller. The first groove extends continuously in a circumference direction from an upstream end to a downstream end. A second groove is formed in a back surface of the casing in an area directly facing the group of concavities in the lower face of the impeller. The second groove extends continuously in a circumference direction from an upstream end to a downstream end. An intake hole and a discharge hole is formed in the casing. The upstream end of the first groove communicates with the outside of the casing via the intake hole. The downstream end of the second groove communicates with the outside of the casing via the discharge hole.
When the impeller rotates, the fuel pump draws fuel into the casing from the intake hole. The drawn fuel flows along the grooves and the concavities of the impeller. Swirl flow occurs between the concavities on the front face of the impeller and the first groove, and between the concavities on the back face of the impeller and the second groove as a result of centrifugal forces caused by the rotation of the impeller. The fuel drawn into the casing flows to the downstream side along the first groove and second groove while creating swirl flow. The fuel is pressurized and the pressurized fuel is discharged out of the casing through the discharge hole.
In the above described fuel pump, the fuel drawn into the casing flows along the first groove and the second groove, and flows from the downstream end of the second groove to the discharge hole. The fuel in the front face (i.e.., downstream side face) of the partition walls of the impeller has nowhere to go when the partition walls of the impeller pass by the downstream end of the second groove (discharge hole), which causes a drastic increase in fuel pressure. As a result of this pressure increase is generated noise of a frequency corresponding to the number of partition walls and to the number of rotations of the impeller.
Japanese Laid-open Patent Publication No. 6-228831 discloses fuel pump in which buffering portion for dampening the rise in fuel pressure are formed in the downstream end of the second groove. This fuel pump has a circular discharge hole provided in the vicinity of the downstream end of a second groove, and buffering groove (i.e.., buffering portion) extending beyond the discharge hole in the rotation direction of the impeller. In this fuel pump, when the partition wall of the impeller pass by the downstream end of the second groove (discharge hole), part of the fuel in the front face of the partition wall can flow to the discharge hole via the buffering groove. As a result, the rise in fuel pressure is dampened, which should result in noise reduction.
In the conventional fuel pumps described above, however, the fuel flowing from the buffering groove towards the discharge hole collides with the fuel flowing from the second groove towards the discharge hole. This hampers the flow of the fuel from the buffering groove towards the discharge hole, which in turn precludes achieving sufficient dampening of the rise in fuel pressure on the front face of the partition walls of the impeller.
In the above conventional fuel pumps, in particular, the discharge hole is shaped with a circular form having a diameter substantially identical to the width of the concavities of the impeller (i.e.., length of the partition walls); as a result, the fuel flowing in the second groove hits against the downstream end of the second groove with substantially identical timing. Fuel flow from the buffering groove towards the discharge hole becomes thereby greatly hindered, which precludes achieving sufficient dampening of the rise in fuel pressure.